Much ado about Pluto

Much ado about Pluto

Recent discoveries of Pluto-like objects in the outer
solar system have sparked debate about the nature of the tiniest
"planet"

Feb 17, 1999: In 1979 the
Solar System became a bit mixed up. That's when Pluto, which
travels in a highly elliptical orbit, temporarily moved closer
to the sun than Neptune. Every 248 years the two planets swap
places and for about 20 years Pluto becomes the eighth planet
and Neptune the ninth. This topsy-turvy situation was rectified
last Thursday, Feb. 11, when Pluto crossed Neptune's orbit and
became the ninth planet once again.

Right: Pluto is the only planet that has not been visited
by a spacecraft. Even the Hubble Space Telescope can resolve
only the largest features on its surface, shown in this image
of one hemisphere. The brightness variations could be due to
craters and basins, methane and nitrogen frosts, or even areas
of primordial organic matter. No one knows, and the mystery may
remain until some future spacecraft pays a visit to this distant
planet. More
information

One of these things is not like the others

But is Pluto really a planet? That's what astronomers have
been discussing since late last year when some members of the
International Astronomical Union suggested that Pluto be given
a minor planet designation. Why? For one thing Pluto is very
small. It's 6 times smaller than Earth, and even smaller than
seven of the solar system's moons (the Moon, Io, Europa, Ganymede,
Callisto, Titan and Triton). Pluto's own moon, Charon, is larger
in proportion to its planet than any other satellite in the solar
system. Some astronomers consider the pair to be a double planet.

Pluto's elliptical orbit is also unusual. It is the only planetary
orbit which crosses that of another planet (Neptune), and it
is tilted 17 degress with respect to the plane of the solar system.
Astronomers once thought that Pluto may have been a satellite
of Neptune's that was ejected to follow a tilted elliptical path
around the sun. However, careful simulations of the orbits and
dynamics of Pluto and Neptune indicate that this is an unlikely
scenario.

Pluto's composition is unknown, but its density (about 2 gm/cm3)
indicates that it is probably a mixture of rock and ice. All
the other rocky planets -- Mercury, Venus, Earth and Mars --
are located in the inner solar system, close to the Sun. Except
for Pluto, all of the outer planets -- Jupiter, Saturn, Uranus
and Neptune -- are gaseous giants. Once again, Pluto is a misfit.

Pluto behind the 8 ball

Despite its well-known peculiarities, Pluto's official status
as a planet was never in jeopardy until 1992 when David Jewitt
and J. Luu discovered a curious object called 1992 QB1. QB1 is
a small icy body, similar in size to an asteroid, orbiting 1.5
times further from the sun than Neptune. QB1 was the first hint
that there might be more than just Pluto in the distant reaches
of the solar system.

Right: Kuiper Belt object 1995 WY2
observed UT 1995 Nov 18 using the Keck 10 meter telescope by
D. Jewitt (University of Hawaii) and J. Luu (University of California,
Berkeley). These are the discovery images. Each is a 900 second
integration. The images are separated by about 18 minutes, during
which time the object moves 1 arcsecond. Stationary background
galaxies are also evident in this short movie. The apparent magnitude
is V = 24.2. For more information see David Jewitt's Kuiper
Belt Home Page.

Since then nearly 100 objects like QB1 have been found. They
are thought to be similar to Pluto in composition and, like Pluto,
many orbit the sun in a 3:2 resonance with Neptune. This swarm
of Pluto-like objects beyond Neptune is known as the Kuiper Belt,
after Gerard Kuiper, who first proposed that such a belt existed
and served as a source of short period comets. Astronomers estimate
that there are at least 35,000 Kuiper Belt objects greater than
100 km in diameter, which is several hundred times the number
(and mass) of similar sized objects in the main asteroid belt.

So, is Pluto really a planet or is it more like a dormant comet,
simply the largest known member of the Kuiper Belt? That's the
question that astronomers have recently been debating.

Other than its relatively large size, Pluto is practically indistinguishable
from the Kuiper Belt objects (KBOs) and short period comets.
The main difference is Pluto's reflectivity, which is much higher
than that of known KBOs.
"Pluto has a higher albedo (60%) than we suspect for the
other KBOs," explains Dr. David Jewitt of the University
of Hawaii. "But this is an artifact of size - Pluto has
enough mass & gravity to retain a tenuous atmosphere from which
bright surface frosts may be deposited on the surface."

David Jewitt and his colleagues at the Institute for Astronomy
are leaders in the search for new members of the Kuiper Belt.
They are presently scanning a 50 sq. degree area of the sky for
KBO's using the University of Hawaii's 8192 x 8192 pixel CCD
camera, and are experimenting with an even larger 12,000 x 8,000
pixel camera for faster searches. So far their efforts have met
with considerable success. They have discovered over 40 KBO's
in recent years, some of which are comparable in size to Pluto.

"We've already found objects 1/3rd the diameter of Pluto,"
says David Jewitt," even though we have examined only a
tiny fraction of the sky. An example is 1996 TO66, which is 800
km diameter. It would be incredible in its own right if Pluto
proved to be the only 2000 km object. I think we'll have Pluto
II, Pluto III....within a few years."

Right: Artist's conception of the
Pluto-Kuiper
Express, a spacecraft that NASA planners hope will visit
Pluto and Charon around 2010. Pluto is the only planet in our
Solar System not yet viewed close-up by spacecraft. As a result
many basic questions about Pluto remain unanswered. For example,
what are the mysterious dark patches revealed in Hubble images
of Pluto? Could they be due to photochemical reactions caused
by cosmic rays, or perhaps areas of primordial organic matter?
Does Charon, like Pluto, have dark spots and an atmosphere? Or
is it a very different kind of body? Mission planners hope to
answer these questions and many more. Depending on the success
of the Pluto-Charon encounter, the space probe could go on to
visit objects in the Kuiper Belt.

Nine's a charm

Dr. Jewitt raises the interesting possibility that Kupier
Belt objects might one day be discovered that are even larger
than our ninth planet. If that happens, what does it mean for
Pluto? Should it be stripped of planetary status and reclassified
as a member of the Kuiper Belt? Or should newly discovered "Plutos"
be classified as planets as well?

Left: A
Hubble telescope image of Pluto and its satellite Charon.

These are difficult questions that await the astronomical community.
For now, however, Pluto's status as a planet seems secure. In
a press release dated Feb. 3, 1999 the International Astronomical
Union stated that "No proposal to change the status of Pluto
as the ninth planet in the solar system has been made by any
Division, Commission or Working Group of the IAU responsible
for solar system science. Lately, a substantial number of smaller
objects have been discovered in the outer solar system, beyond
Neptune, with orbits and possibly other properties similar to
those of Pluto. It has been proposed to assign Pluto a number
in a technical catalogue or list of such Trans-Neptunian Objects
(TNOs) so that observations and computations concerning these
objects can be conveniently collated. This process was explicitly
designed to not change Pluto's status as a planet."

Mistaken reports that the IAU intended to strip Pluto of its
planetary status caused an uproar among astronomers and in the
popular press. It seems that Pluto is a sentimental favorite
to remain a planet among both scientists and the public. However,
if more trans-Neptunian objects are discovered that are even
larger than Pluto, the debate could begin anew.